1. Technical Field
The present invention relates to an optical element such as a lens and a filter, an imaging optical system using the optical element for a part thereof, and a camera module in which a lens unit formed by incorporating the imaging optical system into a lens barrel and an imaging unit provided with a solid state imaging element are integrally combined.
2. Related Art
Digital cameras are provided with a CCD or CMOS imaging element and an imaging optical system forming an optical image of a photography subject on an imaging surface of the imaging element. The imaging element is configured as an imaging unit mounted on a circuit board with IC chips for driving the imaging element or processing imaging signals and the imaging optical system is generally configured as a lens unit incorporating an imaging lens into a barrel. In general digital cameras, the imaging unit and the lens unit are individually mounted on a camera case. Meanwhile, the digital cameras are widely built into mobile apparatuses such as mobile phones and PDAs, and such built-in digital cameras are required to be compact. In such built-in digital cameras, the lens unit and the imaging unit are connected to each other to be a module, and the camera module is incorporated into a needed part of a main case of a mobile apparatus.
Recently, the above-described camera module digital cameras are further required to be compact and have high definition. To be high definition, it is not only necessary to increase the number of pixels of the imaging element but also to thereby improve the optical performance of the imaging optical system. It is also indispensable to prevent image quality from decreasing due to flare or ghost. To reduce flare and ghost, a part of flange surface of a lens, is coated with ink to suppress internal reflection so that undesirable light incident to the non-effective diameter area of a part of lenses constituting the imaging optical system or the outer peripheral face of the lens does not reach an effective screen area of the imaging element due to the suppressing of the internal reflection. In addition, a rough face or unevenness is formed on the needed part, and a light shielding plate is additionally provided. Such a method is disclosed in Patent Document 1 (JP-A-2008-175992 corresponds to US-A-2008/0170297) and Patent Document 2 (JP-A-2005-309289).
However, when the undesirable light is incident from the inside to the non-effective diameter area or the outer peripheral face of the lens constituting the imaging optical system and the incident angle of the undesirable light is small and has sufficient intensity, it is difficult to attenuate the specular reflection light to the extent that there is no practical problems even when the incident face is coated with ink and is subjected to a rough face process. In addition, there is a case where the light is reflected with intensity to some extent and returns into the imaging optical system.
FIG. 8 shows an example thereof, an undesirable light W represented by a broken line is incident to a first lens 3 through an aperture diaphragm 2 with an incident angle larger than that of an imaging light G which is effective for imaging. The undesirable light W incident to the first lens 3 with the large incident angle travels toward a circumference of a second lens 4, the undesirable light W is internally and totally reflected by a surface close to an image plane of the second lens 4 with a convex surface facing an object side, particularly, at that part, and the undesirable light W is traveling toward an outer peripheral face 4a of the second lens 4 without substantially attenuating in intensity. The incident angle to the first lens 3 is set to be smaller than that of the undesirable light W, and thus the imaging light G is incident to an imaging element 8 without any problems even when the imaging light G is incident from the circumferential part of the first lens 3 as shown.
The undesirable light W, which travels toward the outer peripheral face 4a as described above, is incident to the outer peripheral face 4a with the small angle, and thus undesirable light in an angle range close to specular reflection is internally reflected to the second lens 4 again with intensity to some extent even when reflectance of the outer peripheral face 4a is suppressed to be sufficiently low by the rough face process or ink coating process. When a plastic formed lens is used as the second lens 4 or a formed lens made by pressing a preform made of heated glass is used, there are many cases where the rough face process or ink coating process of the outer peripheral face 4a is omitted to lower the cost. In such cases, most of the undesirable light incident to the outer peripheral face 4a passes and is absorbed in a lens barrel, but the undesirable light, which is specularly reflected to the outer peripheral face 4a and travels toward the inside of the second lens 4, still has sufficient intensity. When such undesirable light W is incident to the surface of the second lens 4 on the object side and particularly when a concave surface thereof is facing the object side, the undesirable light W is totally reflected toward an image-plane side as shown and is incident to a third lens 5 with a small angle. Then, the undesirable light passes through a fourth lens 6 and an infrared-light cut filter 7 toward the imaging element 8 and causes ghost or flare.